Measured on Recurring Basis (Details) (USD $)							9 Months Ended
	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2013 Insurance Sector Auction Rate Securities [Member]	Mar. 31, 2013 Insurance Sector Auction Rate Securities [Member]	Dec. 31, 2013 Insurance Sector Auction Rate Securities [Member] Minimum [Member]	Dec. 31, 2013 Insurance Sector Auction Rate Securities [Member] Maximum [Member]	Mar. 31, 2013 Energy And Telecommunications Sector Insurance Sector Auction Rate Securities [Member]	Mar. 31, 2013 Student Loan Auction Rate Securities [Member]	Dec. 31, 2013 Fair Value, Inputs, Level 1 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 1 [Member]	Dec. 31, 2013 Fair Value, Inputs, Level 2 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2013 Fair Value, Inputs, Level 3 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 114,235,000	$ 100,878,000									$ 114,235,000	$ 100,878,000	$ 0	$ 0	$ 0	$ 0
Marketable Equity Securities Fair Value Disclosure		5,509,000										5,509,000		0		0
Corporate Bonds and Debt Fair Value Disclosure	858,638,000	685,122,000									0	0	852,448,000	678,932,000	6,190,000	6,190,000
Government Agency Bonds Fair Value Disclosure	736,011,000	558,153,000									0	0	736,011,000	558,153,000	0	0
Cash Deposit Accounts Fair Value Disclosure	260,865,000	427,456,000									0	0	260,865,000	427,456,000	0	0
Municipal Bonds Fair Value Disclosure	51,751,000	25,138,000									0	0	51,751,000	25,138,000	0	0
Auction Rate Securities Fair Value Disclosure	9,827,000	33,791,000									0	0	0	0	9,827,000	33,791,000
Contingent consideration	0	19,100,000										0		0		19,100,000
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	375,100,000	528,334,000	465,293,000	635,755,000							114,235,000	100,878,000	260,865,000	427,456,000	0	0
Short-term investments	932,302,000	1,050,263,000									0	0	932,302,000	1,050,263,000	0	0
Long-term investments	723,925,000	257,450,000									0	5,509,000	707,908,000	211,960,000	16,017,000	39,981,000
Total assets measured at fair value	2,031,327,000	1,836,047,000									114,235,000	106,387,000	1,901,075,000	1,689,679,000	16,017,000	39,981,000
Liabilities, Fair Value Disclosure		19,100,000										0		0		19,100,000
Available-for-sale Securities, Other Disclosure Items [Abstract]
Value of auction rate securities for which recent auctions were unsuccessful					$ 9,800,000	$ 9,800,000			$ 5,300,000	$ 18,700,000
Discount rate							2.00%	2.50%
Liquidity risk premium (percentage)							9.10%	29.50%
Anticipated liquidity horizon							7 years	10 years